For a wide variety of processes from recycling old rubber, to preparing thermoplastics for molding, to powdering chili peppers, it is desirable to reduce particulate material to very fine mesh. If soft or resilient particles are cooled until they are brittle, they may be efficiently fragmented. To cool the particles to where they are brittle, the particles may be sprayed or soaked in cryogenic liquids in apparatus such as those described in U.S. Pat. Nos. 3,992,899, 3,990,641 and 3,897,010. The cold brittle particles may be ground, e.g., in an impact grinder, into tiny mesh fragments. The ground fragments may be shifted through screens of appropriate mesh to obtain particle fragments of a desirably small size.
A persistant problem with apparatus that cools particles to brittleness and grinds the cold, brittle particles is non-uniformity of cooling and/or subsequent heating of particles in the grinding apparatus whereby non-brittle particles are processed in the grinder. Soft or resilient particles are not adequately fragmented in the grinder and tend to clog up both the grinder and the subsequent screening apparatus. The need exists for improved cooling and mixing apparatus which assures complete cooling of all particles to a temperature at which they will be brittle and from which they will not heat up sufficiently in the grinding apparatus to soften.
In other applications, even where cooling is not necessary to maintain the material being ground in a brittle state, it is desirable to introduce the material into the grinder at a very low temperature to prevent deterioration of the product. For example, it is desirable to precool freshly roasted coffee beans to 40.degree. C. prior to grinding so that the grinding process does not heat the beans sufficiently to result in deterioration of the oil or escape of aromatic substance to the atmosphere.
Apparatus for dispersing particulate material having an outer mixing or intromitter auger rotatable around a central feed auger have been described previously in U.S. Pat. Nos. 3,186,692 and 3,439,836. The relative speed of the two augers is adjustable through gear reducer mechanisms to which the augers are commonly linked. When the augers are linked by gear reducer mechanisms, the available relative rotation ratios are normally fixed or limited by the number of gear wheels, and it may be difficult to optimize mixing and feeding conditions. Normally, the intromitter and feed augers of precoolers are driven by variable speed drive motors, but are linked by gears which drive the augers at a fixed ratio of rotation rates, e.g., a 2:1 ratio of the speed of the intromitter auger to the speed of the feed auger. While the speed of the augers can be easily changed by changing the speed of the motor, the ratio of their speeds can be only changed by replacing the linking gears. It is desirable that precooling apparatus have versatility whereby ratios of rotation rates of the augers can be easily adjusted during operation. It may, for example, be desirable to adjust the mixing and feed rates as the temperature in the chamber changes, e.g., after the start-up of operation. It may also be desirable to change the relative rotation of the augers to adjust for variations in the particulate material which is fed into the chamber.